In a typical cellular network, also referred to as a wireless communication system, a User Equipment (UE), communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a device by which a subscriber may access services offered by an operator's network and services outside the operator's network to which the operator's radio access network and core network provide access, e.g. access to the Internet. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. terminal, mobile station, mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The user equipment may be portable, pocket storable, hand held, computer comprised, or vehicle mounted user equipments, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
User equipments are enabled to communicate wirelessly with the network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks and possibly the Internet.
The network covers a geographical area which is divided into cell areas, and may therefore also be referred to as a cellular network. Each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. evolved Node B (eNB), eNodeB, NodeB, B node, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
Multiple Input Multiple Output (MIMO) and beamforming technologies are very important in modern wireless communications systems because they offer the possibility to increase spectrum efficiency and peak rates. Multi-user MIMO has already been standardized in Global System for Mobil Communications (GSM), in a feature called Voice services over Adaptive Multi-user channels on One Slot (VAMOS), which introduced two layer transmission and reception. This technique addresses only Circuit Switched (CS) speech services. Recently, single user MIMO for Enhanced General Packet Radio Service (EGRPS) was proposed as a way to evolve the GSM/Enhanced Data rates for Global Evolution (EDGE) radio access network. A layer refers to a data stream in the context of MIMO. In Multi-user MIMO the base station transmits multiple streams to multiple user equipments. In single-user MIMO, the base station transmits multiple streams on one user equipment.
Multi-standard Radio Frequency (RF) and digital platforms are becoming common in both base stations and user equipments. Moreover, the chipsets of both the base station and the user equipment often support transmission (Tx) and/or reception (Rx) antenna diversity. Hence, hardware support for MIMO and beamforming technologies is already available or expected to be available in the near future as a significant portion of the GSM network and user equipments. Given the scarcity of the radio frequency spectrum and the potential of MIMO and beamforming to increase spectrum efficiency, it is clearly desirable to apply such techniques to speech services in GSM. Moreover, MIMO and beamforming techniques in GSM should be backward compatible, to a very large extent, with the current GSM air interface. This backward compatibility will result in faster time to market and diminished development costs of the features required in both network equipment and user equipment.
A straightforward implementation of MIMO for EGPRS that is to a large extent backwards compatible with the GSM/EDGE air interface is seen in FIG. 1. Each layer is independently coded and modulated according to an EGPRS modulation and coding scheme. Each layer is assigned a different training sequence. The standardized VAMOS training sequence pairs are proposed after a straightforward mapping of the training bit sequence to antipodal 8 Phase-Shift Keying (PSK) symbols. Thus, the transmitter comprises two parallel EGPRS modulators each modulator being fed its own data stream and training sequence.
In FIG. 1, the EGPRS/EGPRS2 transmitter 100 comprises the two parallel EGPRS modulators 101. The input to one of the EGPRS modulators 101 is user code bits for layer 1 and a training sequence for layer 1. The output is a baseband signal for layer 1, which is transmitted through a Tx antenna 103. The input to the other EGPRS modulator 101 is user code bits for layer 2 and a training sequence for layer 2. The output is a baseband signal for layer 2, which is transmitted through another Tx antenna 103.
Closed Loop MIMO in Long Term Evolution (LTE)
Closed loop MIMO technologies have been standardized in LTE. In particular, LTE supports codebook-based precoding. LTE requires the calculation of three feedback quantities at the receiver, namely Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI) and Rank indicator (RI), in order to perform channel adaptation at the transmitter. At least one of the CQI, PMI and RI may be comprised in Channel State Information (CSI). The CQI is used to select a modulation and coding scheme. The PMI is used to select the codebook index. The RI indicates the preferred number of layers, i.e. data streams. Since the coherence time of the radio channel is in the order of a few milliseconds (ms), LTE has been designed to support fast feedback. The aim of closed loop spatial multiplexing transmission modes in LTE is to adapt the transmission to the current (instantaneous) channel conditions. Channel state information just a few sub-frames old (1 sub-frame has duration of 1 ms) may be already obsolete. The periodicity of the feedback loop is configurable and it is typically in the order of a few ms. In LTE, each base station transmits antenna sends Cell specific Reference Signals (CRS) which are used for CQI measurement, PMI and RI estimation, mobility measurements as well as for demodulation of control signaling. There are up to four CRS patterns corresponding to antenna ports from 0 to 3. The CRS patterns on diverse antenna ports are orthogonal to each other in the sense that they do not overlap with each other or with user equipment signals in time or frequency domains. The cell specific reference signals are not precoded.
Precoding may be seen as a generalization of beamforming to support multi-stream/multi-layer transmission in multi-antenna wireless communications. In single-stream beamforming, the same signal is emitted from each of the Tx antennas with appropriate weighting (phase and gain) such that the signal power is maximized at the receiver output. When the receiver has multiple antennas, single-stream beamforming cannot simultaneously maximize the signal level at all of the Rx antennas, so multi-stream transmission may be used. In point-to-point systems, precoding means that multiple data streams are emitted from the Tx antennas with independent and appropriate weightings such that the link throughput is maximized at the receiver output. In multi-user MIMO, the data streams are intended for different user equipments and some measure of the total throughput (e.g., the sum performance or max-min fairness) is maximized.
Reconfigurable Multiple Antennas in the Receiver
Recent measurement campaigns on MIMO channels have revealed the potential benefits in capacity performance that may be obtained by adapting the antenna configuration at the receiver. The results show that choosing the best among three possible antenna configurations at the receiver along 20 m route sections, leads to significant gains. The speed of the receiver did not exceed 30 km/h, and the frequency band was 2.65 GHz. This means that the best antenna configuration was kept fixed during time intervals of 2.4 or longer. It is reasonable to expect that when the measurements had been performed in the 900 MHz band, the same gains would have been obtained by keeping the antenna configuration fixed during time intervals longer than 2.4 s, perhaps up to 7 s.
Spectrum and power efficiencies are of paramount importance in wireless communications. Therefore, it is desirable to implement closed loop beamforming techniques for both circuit switched and Packet Switched (PS) services in GSM. Moreover, when beamforming for these services in GSM is to be standardized and deployed, it is important to ensure that it is designed to maximize the link performance, while maintaining, to a large extent, backwards compatibility with the GSM air interface.
Closed loop techniques such as those used in LTE and other wireless technologies are very promising, but cannot be applied to an enhancement of GSM with beamforming, because the GSM air interface does not support the low latency required by the fast feedback channels. In other words, the gains brought about by the closed loop beamforming techniques standardized in LTE cannot be achieved in a GSM system. Moreover, in LTE the Tx antennas in the base station broadcast cell specific reference signals which are essential for the receiver in order to compute the CQI, PMI and RI. Such signals are not available in the current GSM/EDGE air interface.